The present invention relates to a thermal printer, and more particularly, to the improvement of a transmission mechanism for driving a recording medium winding/feeding rotary body, including a platen roll shaft and a drum shaft, in a thermal printer in which recording is conducted on a sheet of recording paper by placing inks of a plurality of colors on top of another on the recording paper during the rotation of the recording paper in one direction.
In conventional thermal printers of the type having a platen roll on which recording is conducted, which are disclosed by, for example, Japanese Patent Unexamined Publication No. 60-63178, a recording period is determined by detecting the rotation of a motor for driving the platen roll by a counter. More specifically, in case the motor is a DC motor, the rotational angle of a motor for driving the platen roll is detected by an encoder or a frequency generator (FG), and a frequency corresponding to the recording period or a number of signals from the encoder corresponding to the recording period is counted by the counter. A control circuit performs recording upon receipt of a signal from the counter.
In case the motor for driving the platen roll is a pulse motor, a number of motor driving pulses are counted by a counter, and the counter sends a signal to the control circuit once it counts the number of pulses corresponding to the recording period, by which the control circuit performs recording, as in the case of the DC motor.
In the thermal printer of the type described above, no consideration is given to transmission errors of a transmission gear train for driving a platen roll shaft or a drum shaft. That is, when recording is conducted by rotating a sheet or recording paper in one direction and placing inks of a plurality of inks one upon another on the paper each gear in the gear train rotates through an arbitrary angle in a single color recording period, so that irregular feeding caused by the transmission errors in the recording period of one color does not coincide with that caused in the recording period of another color, and misalignment of dots of a plurality of colors occurs, thus generating uneveness of color.
This problem may be solved by absolutely improving the transmission accuracy of the gears. However, in the case the gear train includes a large number of gears, accuracy control of individual gears (including pitch accuracy, tooth shape accuracy and tooth trace accuracy in a widthwise direction of gears in, for example, JIS1) does not assure suppression of variations in the transmission errors and uneveness of colors.
The applicants of this invention disclosed a thermal printer in U.S. Pat. application No. 353,695 or West German Pat. application No. 3,915,598.7. This thermal printer includes a platen roll disposed in opposed relation to a line-type thermal head, two drums located on the axially opposite sides of the platen roll, a plurality of pulleys for each drum provided in the circumferential direction of the drum at predetermined intervals and two rotatably supported pulley holders for supporting the pullup and located on the axially opposite side of the drum, and an endless belt for each drum, adapted to be guided by the pulleys such that the inner side of the belt is in close contact with the outer periphery of the drum except for a portion of the drum (hereinafter referred to as an open portion). In such a thermal printer, a sheet of paper is first wound around the drum with the two lateral edges of the paper caught between the drums and the belts, and the sheet of paper is then rotated in one direction in that state by rotating the drums together with the pulley holders, during which recording is conducted by pressing an ink ribbon and the paper against the platen roll by the thermal head and by heating the thermal head to melt or sublimate the ink ribbon.
The above-described thermal printer has an advantage in that a high quality color printing, which has no misalignment of colors, can be conducted because a sheet of paper is not moved in a reciprocating manner during the printing. During a paper feed operation in which a sheet of paper is wound around the drums and a paper delivery operation in which a sheet of paper is taken off from the drums, only the drums are rotated and the paper is fed or taken off by means of the drums and the belts which are rotatingly driven by the drums. However, the above-described thermal printer suffers from the following problems. Firstly, since a sheet of paper is fed to and taken off from above the open portion of the drums, direction of the rotation of the drums during the paper feed and delivery operations must be reversed. In consequence, in case a sheet of paper is slantwise wound around the drums during the paper feed operation, direction of the rotation of the drums must be reversed in order to take off the paper, making recovery operation difficult. Secondly, in the above thermal printer, a sensor, e.g., a reflection type sensor, for detecting the forward end of the paper is provided in the vicinity of the paper feed port of the drums, and the paper feed operation is switched over to the printing operation after a period of time required for the longest sheet of paper that can be printed to be wound around the drums after the sensor detects the forward end of the paper. Accordingly, in the case of a short sheet of paper, staring of the printing operation is inefficiently delayed. That is, printing speed is reduced. Also, in case printing starts after a period of time corresponding to the length of the paper has elapsed, a sensor for detecting the length of the paper is required and a complicated circuit configuration or an operation depending upon the length of the paper is required, deteriorating the operability. Thirdly, switch-over between the rotation of only the drums during the paper feed and delivery operations and the rotation of the drums and the pulley holders as one unit during the printing is conducted by axially sliding a gear. However, when the gear is caused to slide, it cannot be brought into mesh with another gear smoothly, and this makes automatic selection difficult. Hence, sliding of the gear is operated manually, and this deteriorates operability and reduces printing speed. Fourthly, during the printing, the thermal head is pressed against the elastic platen roll under that pressure which allows the platen roll to slightly undergo deformation. However, since the platen roll is mounted to make the outer peripheral surface thereof in alignment with the outer peripheral surface of each of the drums, the paper will be folded at the opposite ends of the platen roll when the thermal head is pressed against the platen roll. As a result, folding of the paper may occur, adversely affecting paper feeding and printing quality.